1. Field of the Invention
This invention relates to an electrical receptacle unit which senses the temperature of the contact portion between itself and any of various types of electrical plugs, such as the plug of an aquarium illumination device, and outputs a corresponding temperature signal in response to which the supply of electrical power to devices connected with the plug is shut off, and to an electrical extension cord.
2. Description of the Prior Art
Since an overheated electrical plug is a fire hazard, some plugs are equipped with a safety device for shutting off the supply of power from the plug to the associated electrical device when plug temperature rises above the rated level.
For example, Japanese Utility Model Public Disclosure Hei 4-118581 teaches a safety device of this type equipped with a temperature rise preventer which operates to cut off the supply of electric power from a plug when the temperature inside the plug becomes abnormally high owing to poor contact between the plug blades and the slotted terminals of the receptacle into which it is inserted. The device can thus prevent the plug from overheating and starting a fire.
In the prior-art safety devices, the temperature rise preventer for detecting increase in plug temperature during power supply and cutting off the supply of power from the plug when the detected temperature rises over a preset temperature is built into the interior of the plug. However, since electrical plugs are generally small and have little interior space, the inclusion of the temperature rise prevention device is difficult and leads to a complicated configuration.
Another disadvantage of the prior-art devices is that they are unable to detect overheating of the plug or its receptacle caused by the tracking (i.e., the formation of electrically conductive paths on an insulator by electric discharge or leakage) that progressively accumulates on the insulator material of the plug or the receptacle.
The present invention was accomplished for overcoming these shortcomings of the prior art and has as its object to provide an electrical receptacle unit that has adequate space for a built-in temperature sensor, can reliably detect the temperature of a plug inserted therein, cuts off the supply of electric power to the inserted plug when the temperature rises above a preset level owing to tracking or the like, and produces a warning by activating an alarm when the temperature of the inserted plug exceeds the preset temperature.